This invention relates to a system for detecting a fault in a power transmission line having a pair of ends, such as a power station and a substation.
A fault location system is well-known which calculates a distance between either one of the ends of the transmission line and a point of occurrence of the fault. In a conventional fault locating system, use is made of surges which are inevitably developed at the fault point when the fault takes place. The surges are received at both of the ends. From a time difference between the respective instants of reception, the distance is calculated. Another conventional fault locating system supplies, no occurrence of the fault, a sequence of pulses from a particular one of the ends to the transmission line and receives pulses reflected back from the fault point. In this system, a time interval is measured between a time instant when the pulses are generated at the particular end and another time instant when the reflected pulses are received thereat in order to calculate the distance. Thus, the distance is calculated in the conventional systems by the use of the surges and the pulses. The surges and the pulses are susceptible to deformation and attenuation during propagation through the transmission line. Moreover, the surges and the reflected pulses are considerably varied in their waveforms according to aspects of the faults. Therefore, it is difficult with the conventional fault locating systems to precisely detect the fault point.
By the way, it is possible for maintenance to divide a transmission line into a plurality of sections and to resort to a distance relay system comprising a distance relay at one end of the transmission line. With the distance relay, a fault is located in one of the sections by the use of voltages and currents measured directly at the commercial frequency. It is, however, impossible with this system to precisely calculate a distance between the fault point and the end. This is because the calculation is adversely affected by a fault resistance which inevitably appears at the fault point. In addition, it is impossible to calculate the fault resistance. Therefore, it is difficult with this system to analyze aspects and causes of the fault.